1. Field of the Invention
This invention relates to derailleur-type bicycle gear shifting devices. More particularly, it relates to spring biased derailleur-type shifting devices exhibiting a desired biasing force curve.
2. Description of Related Art
Bicycle derailleurs are used to effect gear shifts by changing the position of a drive chain between the variable diameter sprockets of a multi-sprocket freewheel or crankset. The position of a chain on a sprocket of a freewheel or on a chainring of a crankset determines the gear ratio of the bicycle at which the rider must pedal. Typically, a derailleur includes a parallelogram link mechanism that is attached to a bicycle frame element at one end and to a chain guide at another end, the chain guide laterally urging the chain between the multiple sprockets or chainrings. A control cable is tied at one end to a shifter mounted proximate the rider's hand on the handlebar, or on the downtube of a bicycle frame and at the second end to the parallelogram. The parallelogram is typically spring biased in a given direction, requiring the rider to actuate the shifter and tension the control cable in order to deflect the parallelogram, and in turn urge the chain, in a direction opposite to the spring biased direction.
The return spring biasing the parallelogram is typically a helical or coil spring urging together diagonally opposed pivot points of the parallelogram. FIG. 1 illustrates a parallelogram that is deflected from position (a) to position (b) by an actuation of a cable force F.sub.c against the biasing force of a coil return spring. As the parallelogram is rotated or laterally deflected, the spring force increases; however, the moment arm over which it acts decreases from l.sub.a at undeflected position (a) to l.sub.b at deflected position (b). For approximately the first half of parallelogram movement, the spring force is increasing at a rate greater than the spring moment arm is decreasing producing an increasing biasing force F.sub.b. However, for the second half of parallelogram movement, the spring moment arm progressively decreases at a rate greater than that of the still increasing spring force, producing a biasing force. FIG. 2 illustrates the progression of biasing and spring forces and spring moment arm over the actuation range of a typical parallelogram biased by a coil spring. Typically, a rear derailleur is spring biased toward the smaller diameter or higher gear sprocket. Shifting to a lower gear by urging the chain toward the smaller sprocket requires a generally uniform biasing force, but biasing force tension falls off when approaching the largest sprocket or lowest gear where shifting can be most critical.
Known prior art devices have included multiple spring combinations that increase the biasing force of a primary coil spring. Although the increased biasing force of such devices may counter the effects of spring fatigue, loss of lubrication, or contamination of the link mechanism by dirt or grit, the incremental increase in spring force entire range of parallelogram movement thereby failing to counter the effects of spring drop-off towards the latter half of derailleur movement. A need therefore exists for a derailleur producing a substantially constant actuation or biasing force over its range of movement.